1. Field of the Invention
The present invention relates to a lithographic apparatus, a radiation system, a device manufacturing method and a radiation generating method.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
To image smaller features, it has been proposed to use extreme ultraviolet radiation (EUV) with a wavelength in the range of 5-20 nanometers, in particular, 13.5 nanometers, or a charged particle beam, e.g., an ion beam and an electron beam, as the exposure radiation in a lithographic apparatus. These types of radiation need the beam path in the apparatus to be evacuated to avoid absorption. Since there are no known materials for making a refractive optical element for EUV radiation, EUV lithographic apparatus use mirrors in the radiation, illumination and projection systems. Such mirrors are highly susceptible to contamination, thereby reducing their reflectivity and hence the throughput of the apparatus. Further, sources for EUV may produce debris whose entry into the illumination system should be avoided.
In order to reduce the chance of debris entering the illumination system, the use of contaminant traps is known. Such traps are disposed in the radiation system downstream the source. The traps comprise elements that provide a surface on which debris can deposit. Conventional radiation systems may also comprise a collector that collects the radiation beam. It has been found that debris may also deposit on elements in the collector. The deposit of debris on the collector significantly reduces its operational lifetime before it must be cleaned.
A rotation element trap is a specific contaminant trap type comprising a multiple number of elements extending in a radial direction from a common rotation trap axis. During operation of the lithographic apparatus, the rotation element trap rotates in the path of the radiation beam around the rotation trap axis thereby enabling the elements to catch contamination material, typically tin particles. It is also known to provide a ring shaped contaminant catch for receiving contaminant material particles that are ejected from the rotation element trap elements due to centrifugal forces. The received tin particles flow from the catch towards a tin collection vessel.
Another specific contamination trap type is a static element trap that may also be arranged in the path of the radiation beam, e.g., downstream to the rotation element trap.
It is further known to apply an Argon gas barrier in the rotation element trap to counteract an exponential decrease of the collector lifetime. In order to maintain the Argon gas barrier, a relatively small distance is present between the rotation element trap elements and the ring-shaped catch. However, Tin particles being in the liquid phase when traveling towards the catch tend to become solid particles when hitting the catch, thereby accumulating in the small space between the elements and the catch. In order to prevent the tin particles to solidify it is known to heat the catch so that the tin droplets may flow to the tin collection vessel. In practice, it appears that thermal shorts are created between the heated catch and a cooled static element trap arranged downstream to the rotating element trap. The thermal shorts may cause a drop in the catch temperature leading to a source interlock. Further it appears that the static element trap may crash due to the presence of tin droplets. Therefore, in order to counteract the above-mentioned effects, the tin particles must be frequently removed from the catch and from the static element trap thus causing an undesired increase in downtime of the radiation system.